In an air-outlet mode selecting mechanism, air outlet opening portions such as a face opening portion, a defroster opening portion and a foot opening portion are opened and closed by the use of a single rotary door having an outer peripheral door surface turning with a rotary shaft. Further, the outer peripheral door surface turning with the rotary shaft is arranged at a position separated by a predetermined distance outward in a radial direction from the center of the rotary shaft. However, in this case, all of the face opening portion, the defroster opening portion and the foot opening portion of an air conditioning unit need to be arranged in an arc shape along the turning path of the outer peripheral door surface of the rotary door. Generally, the air conditioning unit is arranged in a place that is inside the instrument board (i.e., dashboard) of the vehicle and is subjected to extreme spatial constraint. Thus, it is difficult to perform an arrangement of the face opening portion, the defroster opening portion and the foot opening portion in the shape of an arc. Further, in order to ensure the respective air volumes blown off from the three air outlet opening portions, the areas of the respective air outlet opening portions need to be enlarged and hence the area of the outer peripheral door surface of the rotary door needs to be enlarged. As a result, the rotary door is increased in size, and the mounting performance of the air conditioning unit in the vehicle is further deteriorated.
To overcome the problems, Japanese Patent Application No. 2002-321267 proposes an air-outlet mode switching device including two rotary doors. Further, in a foot mode, one of the rotary doors is rotated at an intermediate opening-degree position for introducing conditioned air into the defroster opening portion. However, according to experiments by the inventor of the present application, an air amount blown from the defroster opening portion in the foot mode may be greatly changed due to a change of the intermediate opening-degree position of the one rotary door.